Apparatus for producing a very thin mist of power and controls for producing thin mist of power

ABSTRACT

An apparatus for generating a very thin mist of powder with a device for measuring the flow of powder from a supply container and controlling the flow of the powder to an atomization chamber, which includes a device for periodically diverting the flow of the powder past the measuring device so as to determine the effects of aging and impurities on the measuring device and to compensate the measuring device for such effects.

The invention relates to a dusting apparatus.

Dusting apparatus of this type, such as are described for example inDE-A-26 37 875, are used in printing machines, in order to providefreshly printed sheets or endless webs with a thin covering of dust,which prevents the sheets from sticking together. If the dustingapparatus dispenses too little powder, then this object is not reliablyachieved, if too much powder is applied to the printed products, thenthis results in an unpleasant feel of the product for the user andincreases the costs for dusting. Sometimes, an excessive layer of powderis also a drawback at the time of subsequent mechanical treatment of theprinted products, since reproducible friction ratios do not exist. In adusting apparatus according to DE-A-26 37 875, the powder requirement isascertained relatively roughly by a sensor, which produces a signalproportional to the operating speed of the printing machine.

Hitherto the actual monitoring of the thickness of the powder coveringapplied by the dusting apparatus to the printed products took place dueto the fact that a black control sheet was allowed to travel through thedusting apparatus and the whitening on the sheet was monitoredoptically. However, this type of control is obviously not possibleduring routine operation, so that in this case careful supervision bythe operator was necessary in order to ascertain both a shortage as wellas an excess of powder, which resulted from variations in the powderproperties (for example increase in the moisture content) and theadjustment of the dusting apparatus in long term use.

My dusting apparatus described hereafter should therefore be developedby the present invention so that a uniform quality of the powder coatingapplied to the printed products is guaranteed even during production.This object is achieved according to the invention by a dustingapparatus according to claim 1. In the dusting apparatus according tothe invention, the density of a powder stream falling freely from thedosing device into the atomization device is measured. This density isas a whole still relatively high, so that when using less sensitivemeasuring devices, a great variation of the measurement signal isobtained. If one were to measure variations of density directly in themist produced, which is obtained by atomization of the powder in acarrier gas, one would have to use very sensitive measuring devices,which in addition still react very sensitively to interference effects.

Since, in the dusting apparatus according to the invention, a relativemovement between the free-falling powder stream and the measurement gateis carried out at regular intervals, one can use the output signal ofthe measurement gate at those times when the powder stream does not passtherethrough, for the purpose of compensating for interference effectscaused by impurities or ageing. Thus, in particular, it is not necessaryto incorporate a cleaning device in the dusting apparatus, whichcleaning device keeps the measuring section free of impuritiescontinuously or intermittently. Also, temperature influences areobviated in this way.

Apparatus are known per se for determining the suspended particleconcentration in gases, in which the optical parts of the measurementgate are placed at regular intervals in a reference path, which is freefrom suspended particles. Examples of this are EP-A-00 47 049 andCH-A-567 721.

In these known measuring apparatus for determining the suspendedparticle content, either a closed light path or a tube through which apure gas passes, is placed by an electric motor at regular intervals inthe measuring gate or the measuring gate is shifted accordingly. In thiscase, precise evaluation electronics are required on account of thegenerally only low fluctuations of the optical density. On the otherhand, according to the invention, the density of the powder streamfalling freely into the atomization device from the dosing device ismeasured.

Advantageous developments of the invention are described in theSub-claims.

If one produces the relative movement between the measurement gate andpowder stream according to claim 2, then this takes place with verysimple, mechanical means, which are not susceptible to disturbances. Aflexible hose also disturbs the flow of the powder stream only little,since smooth transition points are present throughout in the directionof flow.

The development of the invention according to another embodiment is anadvantage with regard to a reliable and simple deflection of the freeend of the piece of hose guiding the powder stream, controlled by aprogramme control.

The development of the invention according to another embodimentprevents powder from accumulating on shoulders or projections of theapparatus housing, in the vicinity of the measuring section.

With the development of the invention according to another embodiment itis ensured that the flow of the powder stream over the deflectionsurface leading back to the axis of the apparatus takes place in aparticularly careful and uniform manner, which is once more an advantagewith regard to avoiding accumulations of powder and the formation of avortex before the powder stream enters the atomization device. Auniform, laminar entry of the powder stream into the atomization deviceis however an advantage once again with regard to a homogeneous, uniformmist formation.

In a dusting apparatus according to another embodiment, one always hasexactly the same conditions for the flow of the powder stream,irrespective of whether the powder stream is just passing through themeasuring gate or has just been removed from the latter forre-calibration of the measuring device.

The development of the invention according to another embodiment allows,in a simple manner, the strictly synchronous movement of the transmitterand receiver of the measurement gate, without impeding the powderstream.

With the development of the invention according to another embodiment, aparticularly uniform production of mist is guaranteed.

Also, the development of the invention according to another embodimentis an advantage with regard to a uniform mist formation and with regardto a uniform sucking-in of the powder stream into the atomizationdevice.

According to another embodiment, one can optimize the flow conditions inthe atomization device, upstream and downstream thereof according to therespective specific properties of the powder used and according to therespectively desired flow speed of the mist.

The development of the invention according to another embodiment allowsa uniform distribution of the mist produced at a plurality of dischargenozzles, which are arranged distributed transversely above the printedproduct.

With the developments of the invention according to another embodiment,it is ensured that the sensitivity of the measuring device as a whole iskept substantially constant. Thus, the accuracy of adhering to thedesired mist composition is maintained even over very long periods oftime.

The invention will be described in detail hereafter by means ofembodiments, referring to the drawings, in which:

FIG. 1 is a diagrammatic side view of a dusting apparatus for use in aprinting machine, shown in partial axial section;

FIG. 2 is an axial section through a practical embodiment of a lowerunit of the dusting apparatus shown in FIG. 1, in which the powder isatomized, the density of the powder stream to be atomized is measuredand the mist produced is distributed uniformly to various working lines;

FIG. 3 is a horizontal section through a modified measuring device fordetermining the powder stream supplied to the atomization device;

FIG. 4 is a vertical section through the measuring device according toFIG. 3, on section line IV--IV; and

FIG. 5 is a block circuit diagram of a quantity control of the dustingapparatus according to FIG. 1.

In FIG. 1, the reference numeral 10 designates a storage container,which is filled with a powder material to be applied to printed productsin the form of a thin dust coating, which is not shown in FIG. 1.Current powder materials of this type, which prevent freshly printedproducts from sticking together, are for example maize starch, CaCO₃ andsugar with particle sizes of 10 to 50μ, usually between 15 and 20μ.

The powder material is kept in the fluid state in the storage container10 by a stirrer designated generally by the reference numeral 12. Thestirrer 12 is driven by an electric motor 14.

A metering sleeve 18 is screwed to a discharge nozzle 16 located at thelower end of the storage container 10. This dosing sleeve 18 togetherwith a horizontal dosing plate 20 located therebelow defines a dosinggap 22, whereof the axial dimension can be adjusted by rotating thedosing sleeve 18.

The dosing plate 20 is connected by way of a rod 24 to an oscillatingdrive 26 and is moved to and fro in the horizontal direction by thelatter at a typical frequency of 250 Hz, the amplitude of this movementbeing adjustable. The coarse adjustment of the powder stream removedfrom the storage container 10, determined by way of the size of thedosing gap 22, can be finely adjusted by way of the amplitude of themovement of the dosing plate 20.

The powder stream trickling down over the edges of the dosing plate 20is caught by a hopper 28, which comprises a delivery nozzle 30 at thelower end. Fitted to the latter is the upper end of a flexible hose 32,which leads into a measuring chamber designated generally by thereference numeral 34. The powder stream obtained by way of the hose 32falls freely downwards in the latter, in which case it falls through ameasuring gate, which is formed by a light source 36, for example alight-emitting diode, as well as a light detector 38, for example aphotodiode.

The free lower end of the hose 32 may be deflected within the measuringchamber 34 laterally with respect to the connecting line between thelight source 36 and light detector 38, as will be described in moredetail hereafter with reference to FIG. 2 and for producing thismovement a solenoid 40 is fitted to the measuring chamber 34.

The powder stream emerging downwards from the measuring chamber 34passes into an atomization chamber designated generally by the referencenumeral 42, to which a compressed air stream is supplied by way of asolenoid valve 44. Features of the atomization chamber 42 will likewisebe described in more detail hereafter.

The solenoid valve 44 is connected to the output of a compressor 46.

Located below the atomization chamber 42 is a distributor housing 48, inwhich the thin mist produced from the compressed air and powder streamis distributed to a plurality of working lines 50, which lead to nozzlebars 52 extending at right angles to the plane of the drawing of FIG. 1.The latter apply mist jets 54 substantially homogeneously to the upperside of a fresh printed product.

A digital programme control designated generally by the referencenumeral 58 energizes the electro motor 14, the oscillating drive 26, thesolenoid 40 and the solenoid valve 44. Basic parameters for therespective operation may be fed into the latter by way of an input panel60, in particular the length and width of the sheets to be dusted, thedesired density of the mist directed towards the upper side of theprinted product, basic properties of the powder material to be atomized,speed of movement of the printed product etc.

The programme control 58 furthermore cooperates with a sheet detector62, which responds when the front edge of the printed product reachesthe nozzle bars 52 and is de-energized when the rear edge of the printedproduct has left the nozzle bars 52. The programme control 58 is alsoconnected to the output of a pressure sensor 64, which monitors thedelivery pressure of the compressor 46. The programme control 58 alsoreceives the output signal of a level sensor 66, which is located at thelower end of the storage container 10 and responds when sufficientpowder material is no longer available for further operation of thedusting apparatus.

For regulating a constant powder stream in the atomization chamber, theprogramme control 58 is connected to the output of the light detector 38and adjusts the oscillating amplitude and/or oscillating frequency ofthe dosing plate 20 so that the same optical density, thus also the samematerial density of the powder stream is always maintained in themeasuring section formed by the light source 36 and the light detector38.

As can be seen from FIG. 2, the free lower end of the hose 32 is fixedradially in a ring 68, which is in turn connected by way of a plunger 70to the armature 72 of the solenoid 40.

The measuring chamber 34 has a vertical through-hole 74 having a largediameter, so that the ring 68 is movable so far in the lateral directionthat the powder stream discharged from the lower end of the hose 32laterally by-passes the measuring light barrier formed by the lightsource 36 and the light detector 38.

A head-piece 76 of the atomization chamber 42 has a powder-collectinghopper at the upper end. The latter consists of two hopper sections eachextending over an angle of 180°, namely a first hopper section 78,whereof the wall is aligned at an angle of 45° with respect to the axisof the apparatus, as well as a second hopper section 80 extending moresteeply, aligned at an angle of 25° with respect to the axis of theapparatus.

At the lower end, the two hopper sections 78, 80 lead to a commondelivery cross-section, for which purpose the hopper section 78 isconnected to an axial channel portion 84 by way of a rounded transitionsection 82.

Inserted in the head-piece 76 from the lower end face is a hopper 86widening out conically in the downwards direction. The latter leads to acylindrical through-hole 88, which is provided in a central housing part90 of the atomization chamber 42. A hopper 92 again tapering conicallyadjoins the through-hole 88, which hopper 92 is constructed in the upperend of a base part 94 of the atomization chamber 42.

Located in the chamber defined by the housing parts 76, 90 and 94, withan identical radial spacing on all sides from the through-hole 88 andthe hoppers 86, 92 is a nozzle member designated generally by thereference numeral 96, which comprises a central delivery channel forcompressed air, which is open on the under side. The latter is connectedby way of channels 102 provided in a radial support arm 100, to acompressed air connection opening 104, to which the solenoid valve 44 isconnected.

A venturi tube 108 is inserted in a cylindrical housing bore 106 of thebase part 94, adjoining the hopper 92. It may be fixed in differentaxial positions by a set screw 110.

Adjoining the downstream end of the venturi tube 108 is the distributorhousing 48, in which, starting from the common cross-section of theventuri tube 108, various distributor channels 112 are formed, whichlead to the various working lines 50.

Roughly speaking, the dusting apparatus described so far operates sothat the dosing plate 20 delivers a powder stream to the hopper 28,which stream can be adjusted by its oscillation amplitude and itsoscillation frequency. This powder stream is guided by way cf the hose32 normally to a point located above the measuring light barrier formedby the light source 36 and the light detector 38 and is released there,so that the powder stream passes in free fall through the measuringlight barrier. The output signal of the light detector 38 now gives ameasurement of the density of the powder stream, thus of the quantity ofpowder atomized per unit time. The measured powder stream now passesinto the atomization chamber 42 and is mixed thoroughly and uniformlytherein by the compressed air stream supplied by way of the solenoidvalve 44, due to which a very thin, homogeneous mist is produced.

Since the operating behaviour of the measuring light barrier formed bythe light source 36 and light detector 38 changes in the course of timedue to the deposition of powder and aging, the hose 32 is deflectedlaterally at intervals of approximately 10 seconds by the solenoid 40 tosuch an extent that the powder stream completely bypasses the measuringlight barrier. The output signal of the measuring light barrier thusobtained is a measurement of the contamination and aging of themeasuring light barrier and for further interference effects and canthus be used for compensating for corresponding measurement errors, aswill be described in more detail hereafter.

FIGS. 3 and 4 show a modified measuring chamber 34. Provided in itshousing is an inlet channel 14 for the powder stream and an outletchannel 118 for the powder stream enlarged in the form of a hopper at116. These two aligning channels intersect a transverse, offset housingbore 120, in which a light barrier support 122, constructed in a similarmanner to a thread roller, with two flange parts 124, 126 and a hollowhub part 128 connecting the latter, is mounted to rotate. The flangeparts 124, 126 have bores 130, 132 for receiving the light source 36 orthe light detector 38.

Connecting cables 134, 136 for the light source 36 and the lightdetector 38 are guided through the hollow hub part 128 and an adjoininghollow shaft 138. The latter is moved to and fro by a motor (not shownin detail) by an average amount of 45°, so that the light source 36 andthe light detector 38 can be moved from a position fully intersectingthe path of the powder stream (shown in the drawing) into a referenceposition located laterally outside the path of the powder stream. Inthis latter position, the output signal of the light detector 38 is oncemore used for the compensation of measuring errors caused by impuritiesor ageing.

In the upper part, FIG. 5 shows the control of the solenoid 40 by a freerunning pulse generator 140 and a power amplifier 142. The pulsegenerator 140 produces narrow pulses with a frequency of approximately0.1 Hz, so that the solenoid 40 is energized briefly every 10 secondsand moves the hose 32 and thus the powder stream shown in cross-sectionin FIG. 5 at 144, out of the measuring light barrier defined by thelight source 36 and the light detector 38.

The control terminal of a signal memory 146 also receives the outputsignal of the pulse generator 140, which memory is connected to theoutput of the light detector 38. Thus, the output signal of the lightdetector 38 is read into the signal memory 146 at those instants whenthe powder stream 144 travels past the measuring light barrier.

By way of a reversing amplifier 148, the signal stored in the signalmemory 146 serves for actuating the amplification control terminal of anamplifier 150 with a controllable amplification factor. The input of theamplifier 150 receives the output signal of the light detector 38.

A powder stream signal corrected as regards disturbance effects is thusobtained at the output of the amplifier 150. The latter may also beproduced alternatively according to the variation shown in broken linein FIG. 5:

A division circuit 152 receives the output signal from the pulsegenerator 140 at ar activating terminal, by way of an inverter 154, andthus always operates when the powder stream 144 passes through themeasuring light barrier. The division circuit 152 divides themeasurement signal present at the output of the light detector 38 by thereference signal made available at the output of the signal memory 146,so that one acquires at its output a powder stream signal cleaned-up asa whole with regard to impurities, aging phenomena and otherinterference effects.

The latter is applied to one input of a differential amplifier 156,which at its second input receives a reference signal associated withthe reference density value of the powder stream and thus ultimatelyalso the reference density of the mist produced, which reference signal,in the embodiment in question, is made available by an adjustableresistor 158.

The output signal of the differential amplifier 156 is applied to theamplification control terminal of an amplifier 160 with a controllableamplification factor. Its input is connected to the output of a sineoscillator 162, which normally operates at a frequency of approximately50 Hz.

The output signal of the amplifier 160 is applied to a coil 164, whichtogether with an armature 166 connected to the rod 24 as well as ahelical compression spring 168 biasing the latter, forms the oscillatingdrive 26. The mounting of the armature 166 in the radial direction maybe taken care of for example by an end plate 170 produced from slidingbearing material, of a housing 172 of the oscillating drive 26. It willbe seen that the control circuit illustrated in FIG. 5 operates as awhole so that with an increasing optical density of the powder stream144, it reduces the amplitude of the oscillating drive 26, with adecreasing density of the powder stream 144, it increases the amplitudeof the oscillating drive 26. In this way, the density of the powderstream 144 is kept at the desired reference value determined byadjusting the resistor 158.

Since the operation of the dosing plate 20 also depends on the operatingfrequency of the oscillating drive 26 (inertia effects in the powdervolume lying on the dosing plate 20), one can replace the amplituderegulation, which was just described, completely or partly by aregulation on the basis of frequency, as indicated in FIG. 5 by thebroken line 174.

It will be seen that in the manner described above, one can adjust veryaccurately the density of the mist produced, by using a simple lightdetector, which does not fulfil any high precision requirements, and asimple light source. Since the conveying path of the powder extendsvertically throughout the entire dusting apparatus and the entireconveying path is free from shoulders and edges, there is no danger ofthe formation of deposits.

I claim:
 1. Dusting apparatus which includes in combination(a) acontainer (10) for storing atomizable ground powder that has a firstoutlet (16), (b) a controllable dosing device (18-28) positionedadjacent said outlet (16) to receive powder from said outlet (16) andwhich delivers a free falling powder stream (144) of predeterminedvolume to a second outlet (30), (c) a powder requirement sensor (36, 38)which is mounted to monitor the powder flow from said second outlet (30)and which regulates said dosing device (18-26), (d) an atomizationdevice (42) connected to said second outlet (30) of the dosing chamber(18-28) which forms the fine particles of the powder stream into a mistwhich exits through a third outlet (48), (e) at least one nozzleconnected to said third outlet (48) for discharging the mist-like powderstream formed by said atomization device (42) toward an object (56) tobe dusted, (f) said sensor (34) including(1) a measuring means (36, 38)having a measuring axis through which the powder stream (144) is guidedin free fall and which means measures the stream density, and (2)positioning means (32, 40; 122) for selectively establishing one of tworelative positions between the free falling powder stream (144) and themeasuring means (36, 38) the powder stream (144) intersecting themeasuring means (36, 38) in the first of said two relative positionswhile passing clear of the measuring means (36, 38) in the second ofsaid two relative positions.
 2. Dusting apparatus according to claim 1wherein the positioning means for selectively establishing one of tworelative positions between the free-falling powder stream (144) and themeasuring means (36, 38) comprise a flexible piece of hose (32) having adischarge end that lies above the measuring means (36, 38) and which canbe moved in a direction perpendicular to the measuring axis of themeasuring means (36, 38).
 3. Dusting apparatus according to claim 2wherein the discharge end of the piece of hose (32) is guided radiallyin a ring (68), which is positively connected to an output part of asolenoid (40).
 4. Dusting apparatus according to claim 2 wherein acollecting hopper (28, 80) is provided below a region swept by thepowder stream (144) upon movement of the flexible piece of hose (32). 5.Dusting apparatus according to claim 4 wherein in the peripheraldirection, the collecting hopper comprises two successive wall sections(78, 80) of different inclination with respect to a hopper axis, themore steeply inclined wall section (80) lying below the path of theflexible piece of hose (32).
 6. Dusting apparatus according to claim 5wherein more steeply inclined wall section (80) of the collecting hopperis inclined at an angle of approximately 45° and the less steeplyinclined wall section (80) of the collecting hopper is inclined at anangle of approximately 25° to the axis of the collecting hopper. 7.Dusting apparatus according to claim 1 wherein the positioning meansincludes means (122, 138) for shifting the measuring means (36, 38) in adirection perpendicular to the direction of the powder stream (144). 8.Dusting apparatus according to claim 4 wherein adjoining a lower end ofthe collecting hopper (78, 80; 116) receiving the free-falling powderstream (144) behind the measuring means (36, 38) is an atomizationchamber (86, 88, 92) of the atomization device (42), which comprises afirst chamber section (86) widening out conically, a second chambersection (88) having a constant cross-section as well as a third chambersection (92) tapering conically, and that located at a substantiallyconstant radial distance from walls of the atomization chamber, in itsinterior, is a nozzle member (96), whereof a lower conical end sectioncomprises a central, axial discharge opening (98) for the gaseouscarrier stream.
 9. Dusting apparatus according to claim 8 wherein aventuri insert (108) is disposed below the atomization chamber. 10.Dusting apparatus according to claim 9 wherein the venturi insert (108)is fixed in an axially adjustable manner to a housing of the atomizationdevice (42).
 11. Dusting apparatus according to claim 10 which includesa distributor device (48) located below the venturi insert (108), inwhich a plurality of discharge channels (112) are formed, whichoriginate jointly at a downstream end of the venturi insert (108) andthen diverge.
 12. Dusting apparatus according to claim 1 wherein thedosing device comprises a dosing plate (20) located at a predeterminedaxial distance below the outlet of the storage container, is able toreciprocate perpendicularly to the axis of the outlet at adjustablemomentum by virtue of a momentum control circuit (156 to 160) whichvaries the momentum of the reciprocating movement of the dosing plate(20) inversely to the change of output signal of the measuring means(36, 38).
 13. Dusting apparatus according to claim 1 which includes acontrollable measuring amplifier (150) following the measuring means(36, 38) and by a signal memory (146), also following the measuringmeans (36, 38), which is activated when said second relative positionbetween the measuring means (36, 38) and the free-falling powder stream(144) is established, and whereof an inverted output signal is suppliedto a control terminal of the measuring amplifier (150).
 14. Dustingapparatus according to claim 1 which includes a division circuit (152)following the measuring means (36, 38) and by a signal memory (146) alsofollowing the measuring means (36, 38) which is activated when saidsecond relative position between the measuring means (36, 38) and thefree-falling powder stream (144) is established, inputs of the divisioncircuit (152) being connected to the outputs of the measuring means (36,38) and of the signal memory (146).
 15. Dusting apparatus according toclaim 1 wherein the measuring means (36, 38) comprises a light barrieroperating in transmission.
 16. Dusting apparatus according to claim 1wherein the positioning means includes means (122, 138) for shifting themeasuring means (36, 38) in a direction perpendicular to the directionof the powder stream (144).
 17. Dusting apparatus according to claim 16,wherein a transmitter (36) and a receiver (38) of the measuring meansare disposed in flange parts (124, 126) rigidly connected by a hub part(128), which parts are mounted to rotate in a measuring chamber housing(34).
 18. Dusting apparatus according to claim 16 wherein a collectinghopper (116) receives the free-falling powder stream (144) behind themeasuring means (36, 38).